1. Field of the Invention
This invention pertains to a current detectable transistor equipped with a main transistor in which a main current flows and a sense transistor for detecting a current flowing in the main transistor, and more particularly to a current detectable transistor having a stable sense voltage little affected by a temperature change by providing a resistor whose temperature coefficient is suitable for maintaining the constancy of the sense voltage regardless of temperature change.
2. Description of the Related Arts
A power transistor having a large current flow requires a swift detection of an overcurrent for its protection. For this purpose, a power transistor having an emitter terminal for detecting a current in addition to a sense transistor for detecting a current has been designed and put into a commercial use.
A current detectable transistor equipped with a main transistor in which a main current flows and a sense transistor for detecting the current flowing in the main transistor necessitates a resistance (hereafter referred to as a sense resistance) for obtaining the sense voltage as a voltage drop due to the current flowing in the sense transistor. Ordinarily, a sense resistance without a temperature dependency is used for accurately measuring the sense current value.
FIG. 1 shows an equivalent circuit when a current detectable transistor having a sense resistor is in saturation.
Generally in saturation, a main transistor and a sense transistor can be regarded as two resistances having a certain ratio. Assuming the main transistor and the sense transistor is formed by the plurality of cells, and the ratio of a member of cells of a main transistor to those of a sense transistor is n:1, and R.sub.CE, is the on-resistance (the resistance to current flow in the normal direction between a collector and an emitter) of a main transistor, and the on-resistance of a sense transistor is expressed as nR.sub.CE.
In the equivalent circuit shown in FIG. 1, the on-resistance R.sub.CE of the main transistor between its collector C and its emitter E is connected in parallel with the on-resistance nR.sub.CE, of the sense transistor between the collector C and sense emitter E.sub.S. Further, on-resistance nR.sub.CE, of the sense transistor connects to the sense resistance R.sub.S in series.
Defining a collector current and a sense current respectively as I.sub.C and I.sub.S, the sense voltage V.sub.S can be expressed as: ##EQU1##
Because the on-resistance R.sub.CE, of a semiconductor element is temperature dependent, it can be expressed as EQU R.sub.CE =(1+.alpha..sub.K .DELTA.T)R.sub.CEO ( 2)
where .alpha..sub.K is the temperature coefficient of a semiconductor element, .DELTA.T is a temperature change, and R.sub.CEO is the initial value of on-resistance R.sub.CE.
Because the sense resistance R.sub.S is not temperature dependent, sense voltage V.sub.S can be expressed as ##EQU2##
As described above, even though the sense resistance R.sub.S is not temperature dependent, on-resistance R.sub.CE is temperature dependent. Therefore, as is evident from above expression (3), a temperature change causes the sense voltage V.sub.S to change. Consequently, the value of an overcurrent cannot be determined accurately, thereby preventing a current from being detected with high precision.
A solution to this problem has been considered, in which a separate resistance is provided for compensating for the change of the sense resistance R.sub.S due to temperature change, thereby making the sense voltage V.sub.S constant regardless of a temperature change. However, because this solution invites an increase in the number of parts, it is contrary to the need for miniaturizing the transistor.